a) Field of the Invention
The present invention relates to a method for treating a liquid containing non-soluble particles in suspension in order to separate these particles from the liquid.
The invention also relates to an apparatus hereinafter called "clarifier", for use to carry out this method.
In the following disclosure, reference will be made almost exclusively to waste water as an example of such a liquid containing non-soluble particles that can be treated by the method according to the invention. However, it must be understood that the invention is not restricted to the treatment of waste water, as it could actually be used to treat any other kind of liquid in order to remove from the same solid particles that are in suspension. Such other liquid could be petroleum, oil, chemicals. etc. . .
b) Brief Description of the Prior Art
It is of common practice in numerous industries, especially in the pulp and paper industry to treat the waste water that is generated within the plants in order to separate the non-soluble particles that may be in suspension in it prior to discarding and/or recycling the waste water.
Usually, such a treatment is carried out in one or more apparatuses called "clarifiers" or "dissolved air-floatation units". As is shown in FIG. 1 identified as "prior art", the known clarifiers comprise a tank 1 in which the liquid to be treated is fed through a supply duct 3. Part of the liquid passing through the duct 3 is derived from the same via a line 4, and is saturated with air or any other gas under pressure prior to being reinjected into the supply duct by means of a pump 5. Alternatively, part of the clarified liquid leaving the tank 1 is derived from the outlet of the same via a line 4' and is saturated with air prior to being injected into and mixed with the liquid fed through the duct 3. In both cases, as soon as the air-saturated liquid enters the duct 3, it is subject to a depressurization which generates gas bubbles of a few micrometers within the tank. Such micro-bubbles adhere to the particles in suspension in the liquid and form "flocs" that are buoyant, thereby forming a floating layer of sludge 7 that is mechanically skimmed off from the tank with a scraper 9. The remaining, "clarified" liquid is removed form the tank via a plurality of liquid outlets 11 that are located in a pipe extending close to the bottom of the tank.
To improve the efficiency of the clarifier, it is of common practice to mount a plurality of upwardly inclined plates or baffles 13 in parallel relationship within the tank in order to divide the same into a plurality of upwardly inclined channels having upper ends at a short distance from under the floating layer of sludge 7, and lower ends close to or opening directly into the liquid outlets 11. Such a division of the liquid supplied into the tank into a plurality of separate channels increases the surface separation of the clarifier and reduces the flow to surface ratio (also called "overflow rate") within the clarifier, thereby favorizing separation of the particles.
In this connection, it can be understood that within each channel defined by a pair of plates, viz. an "upper" one and a "lower" one, both extending at the same inclined angle, the small particles lifted up by the microbubbles have time to move up to the upper plate and then to "slide" up along this upper plate while they agglomerate into larger particles. In the meantime, the liquid may slowly move down towards the outlet 11. Such a counterflow motion within each channel is schematically exemplified with dots representative of the particles, and with arrows in one of channels identified by letter "A" in FIG. 1.
To improve the efficiency of the clarifier, it is also of common practice to add polymeric additives to the liquid fed within the tank in order to agglomerate the particles into larger particles, thereby improving their floatation.
In use, the existing clarifiers are quite efficient and are used for the treatment of waste water containing up to 1% or more of particles in suspension with removal of about 10 to 80 kg of particles per m.sup.2 and per hour.
However, the existing clarifiers also have drawbacks because of their "horizontal" structure, they require a substantial amount of space that makes them cumbersome and difficult to install in existing premises.
Secondly, whatever be the amount and pressure of the air injected into the waste water prior to its introduction in to the tank, the generation of micro-bubbles within the tank is difficult to control and adjust because fast and sudden depressurization occurs as soon as the waste water enters the tank at one end thereof, thereby causing most the air bubbles to be generated and to move up at this one end without necessarily adhering to particles, thereby reducing the overall efficiency of the clarifier.